Process for the production of ceramic materials

ABSTRACT

A process for the production of α&#39; or β&#39;SiAlON from a starting mixture of silicon metal and clay, wherein the process includes heating the mixture in a flowing nitrogen, or nitrogen containing, atmosphere to a temperature sufficient to react the silicon and the nitrogen with the clay to form the β&#39; or α&#39;SiAlON and wherein the clay participates in the reaction as a source of aluminium and silicon.

TECHNICAL FIELD

The invention relates to a process for the production of SiAION's and inparticular to the production of β'SiAlON, α'SiAlON, and composites withthese.

BACKGROUND ART

The term SiAlON, or silicon aluminium oxynitride, encompasses a familyof compounds or phases comprised of the elements: silicon, aluminium,oxygen and nitrogen. Each phase is described by a composition range forwhich that particular structure is stable. β'-phase SiAlON (β'SiAlON) isstable over the composition range: Si_(6-z) Al_(z) O_(z) N_(8-z) wherez=0 to 4.2. This includes silicon nitride (βSi₃ N₄) as the z=0 endmember. β'SiAlON has the same structure as silicon nitride (βSi₃ N₄),and can be regarded as a solid solution formed by substituting equalamounts of aluminium and oxygen for silicon and nitrogen respectivelyinto the silicon nitride structure. The amounts of aluminium and oxygenwhich can be substituted into this structure increase with temperature.At 1750° C., z can range from 0 to 4.2. In general terms β'SiAlONcompositions can be referred as low z compositions and high zcompositions with low z being <3 and high z being ≧3. The z valuebasically refers to the aluminium content of the composition.

α'-phase SiAlON (α'SiAlON) has a structure derived from αSi₃ N₄ which isstabilised by metal cation (M) such as Y, Li, Ca. The general formula is

    M.sub.m/v Si.sub.12-(m+n) Al.sub.m+n O.sub.n N.sub.16-n

where m and n indicate the replacement of (m+N) (Si--N) bonds bym(Al--N) and n(Al--O) bonds and v represents the valency of the metalcation M. The limits of the αSiAlON composition are restricted and varywith the size and nature of the stabilising cation. For example, thelimits of solubility of yttrium have been found to vary m/v from 0.33 to0.67 for one composition range. A limited range of metal cationsstabilise the αSiAlON structure. These are Li, Ca, Mg, Y and a number ofthe rare earth metals but not La or Ce. SiAlONs are advanced ceramicmaterials which exhibit useful properties such as high strength andhardness, low density, wear resistance and corrosion resistance, and areable to retain these properties at high temperatures. α'SiAlON, whenfully dense, is a very hard material but brittle. β'SiAlON is less hardbut tough. A composite of the two is a good compromise and yieldsexcellent mechanical strength and wear resistance. SiAlONs are used inrefractories and for a variety of engineering applications such ascutting tools, spray nozzles and pump seals. The exact properties of agiven SiAlON depend on the chemical composition and fabricationvariables, such as purity, grain size and shape, and the method offabrication. β'SiAlON has similar properties to silicon nitride whichinclude excellent resistance to attack by molten metal. Silicon nitrideis commonly used as a refractory material.

Documents indicating the state of the art include:

U.S. Pat. No. 3,960,581 to Ivan B Cutler discloses a process for makingSiAlONs from readily available raw materials, such as clay, togetherwith carbon. There is no teaching or recognition however of the use ofsilicon metal in the process. Use of silicon metal allows synthesis oflow z SiAlON compositions. In addition, the use of carbon is preferable,but not essential, in the process of the present invention. It is alsolow z β'SiAlON compositions that allow the formation of α'SiAlONcompositions by the process of the present invention as will be furtherdescribed herein.

DD 263749 to Akad Wissenschaft DDR, inventor Schikore H, which describesthe production of SiAlON-based materials from a charge containing byweight (A) 75-95% clay, 5-25% carbon, and 0-50% aluminium compounds; or(B) 50-80% clay, 20-50% silicon carbide, and 0-50% aluminium compounds.No disclosure of the use of silicon metal is made and carbon or siliconcarbide must be used.

U.S. Pat. No. 4,360,506, inventor Paris R A, which discloses theformation of β'SiAlON's from a paste comprising silico-aluminousmaterial (clay), carbon, and fine particles of a ligneous material (egsawdust). The carbon and ligneous material are essential and no mentionof the silicon metal is made.

U.S. Pat. No. 4,871,698, inventors Fishler et al, uses silicon metal inthe production of a refractory body. The other constituents includecarbon, β'-SiAlON, clay, silica and silicon carbide amongst others.

Other common methods for producing α'SiAlON's and β'SiAlON's include:

(i) Reaction Sintering of mixtures of two or more of the following: Si₃N₄, SiO₂, Al₂ O₃, AlN, and AlN-polytypoids, at ≧1600° C. under anitrogen atmosphere, usually in the presence of a rare earth sinteringaid such as Y₂ O₃ or CeO. This process involves expensive raw materialsand high temperatures, but allows good control over the composition andpurity of the product.

(ii) Carbothermal Reduction. Aluminosilicate minerals are blended withcarbon and fired at ≧1350° C. under a flowing nitrogen atmosphere. Thisprocess is described as carbothermal reduction because the carbon actsby reducing the aluminosilicate, allowing nitridation to occur. Thisprocess involves cheap raw materials and lower firing temperatures thanfor reaction sintering but impurities in the aluminosilicate can degradethe properties of the product. The process is more difficult to controlbecause it involves stopping a reaction at a specific point prior tocompletion.

(iii) Combustion Synthesis. A mixture containing silicon metal powder isignited under a nitrogen atmosphere. The energy evolved by the stronglyexothermic nitridation of silicon propagates a reaction front throughthe reaction mixture. This method is very rapid and energy efficient butis difficult to control.

Methods (ii) and (iii) both yield β'SiAlON powders which must then beformed and sintered to obtain a ceramic body. Method (i) is the mostcommonly used method for preparing α and β'SiAlON. As is apparent fromabove known methods, in order to get good control over the compositionand purity of the product expensive raw materials and/or extremereaction conditions are required.

It is an object of the invention to provide an improved process for theproduction of α' and β'SiAlONs.

SUMMARY OF THE INVENTION

The invention in a first aspect comprises a process for the productionof α' or β'SiAlON from a starting mixture of silicon metal and clay,wherein the process comprises heating the mixture components in aflowing nitrogen, or nitrogen containing, atmosphere to a temperaturesufficient to react the silicon and the nitrogen with the clay to formthe β' or α'SiAlON and wherein the clay participates in the reaction asa source of aluminium and silicon.

The invention in a second aspect comprises a process for the productionof α' or β'SiAlON from a starting mixture of silicon metal, clay andnitrogen wherein the process comprises dehydroxylating the clay, mixingthe dehydroxylated clay with the silicon metal and heating thecombination in a flowing nitrogen, or nitrogen containing, atmosphere toa temperature sufficient to react the mixture to form α' or β'SiAlON andwherein the clay participates in the reaction as a source of aluminiumand silicon.

Preferably the clay content in the starting mixture is between 11 and85% by weight, more preferably between 11 and 80% by weight and morepreferably between 20 and 70% by weight.

Preferably the β'SiAlON formed by the process of the invention is withinthe composition range:

    Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z

where z is in the range of 0.1-3.0.

Preferably z is between 0.5 and 2.5 and more preferably between 0.5 and1.5.

Preferably the α'SiAlON formed by the process of the invention has acomposition characterised by the general formula:

    M.sub.m/v Si.sub.(m+n) Al.sub.(m+n) O.sub.n N.sub.16-n

where M is a metal cation having a valence v and where m and n indicatethe replacement of (m+n) (Si--N) bonds by m(Al--N) and n(Al--O) bonds inthe α-Si₃ N₄ structure.

Preferably the α'SiAlON is formed from β'SiAlON having a z range between0.1 and 2.0, and more preferably between 0.5 and 1.5.

Preferably the starting mixture further includes additives selected fromcarbon and silicon carbide.

Preferably the starting mixture further includes sintering aids selectedfrom yttria, calcia, magnesia and/or lithia.

Preferably the clay contains a free silica component.

Preferably the atmosphere is substantially pure nitrogen, ahydrogen/nitrogen mixture, or ammonia.

Preferably the components of the mixture are present as fine powders.

Preferably the starting mixture contains, by weight, about 11% to about80% clay, about 9% to about 89% silicon metal and 0% to about 20%carbon.

Preferably the flowing N₂ atmosphere comprises about ≦0.5% oxygen andabout ≦0.5% water vapour.

Preferably the mixture is heated to between about 1100° C. and about1900° C., more preferably between about 1350° C. and about 1900° C.,more preferably between about 1400° C. and about 1750° C. and mostpreferably 1450° C.

Preferably the components are heated at a rate of between substantiallyabout 1° C. and about 20° C. per minute, more preferably between 1° C.and 10° C. per minute, more preferably between about 1° C. and about 5°C. per minute. more preferably between about 1.5° C. and about 2.5° C.per minute and most preferably at about 2° C. per minute.

Preferably the components are held at the required temperature for up toabout 12 hours and most preferably for up to about 8 hours.

Preferably the clay is a hydrated clay mineral and more preferably aKaolin clay.

Preferably the clay is dehydroxylated prior to mixing with the siliconmetal.

Preferably a ceramic material is included in the component mixture.

Preferably the ceramic material included in the mixture is selected fromsilicon carbide (SiC), alumina (Al₂ O₃), aluminium nitride (AlN),silicon nitride (Si₃ N₄), SiAlON, zirconia (ZrO₂) or silica (SiO₂).

Preferably the ceramic material included in the mixture will be coarserthan the other mixture components which react to form the α' orβ'SiAlON.

Preferably the ceramic material included in the mixture will constituteup to about 75% by weight of the mixture, and more preferably betweenabout 40% and about 70% by weight of the mixture.

In more limited terms the invention in a third aspect comprises aprocess for the production of β' or α'SiAlON from a fine powdercomponent mixture comprising substantially, by weight, 50% to 70%silicon metal, 20% to 40% clay, and 5 to 10% carbon, wherein the clayparticipates in the reaction as a source of aluminium and silicon, theprocess comprising the steps of:

a) heating the components at a rate of about 1.5° C. to about 10° C. perminute, to a temperature of about 1350° C. to 1900° C. under a flowingN₂ atmosphere having about ≦0.5% oxygen and about ≦0.5% water vapour;

(b) holding the temperature between about 1350° C. and about 1900° C.for up to about 8 hours; and

(c) recovering the formed product.

In more limited terms the invention in a fourth aspect comprises aprocess for the production of a composite ceramic including β' orα'SiAlON from a mixture of fine powder components comprising, by weight,up to about 75% of a ceramic material and up to about 25% of a β orα'SiAlON forming mixture, wherein the β' or α'SiAlON forming mixturecomprises substantially, by weight, 50% to 70% silicon metal, 20 to 40%clay, and 5 to 10% carbon, wherein the clay participates in the reactionas a source of aluminium and silicon the process comprising the stepsof:

(a) heating the components at a rate of about 1.5° C. to about 10° C.per minute, to a temperature of substantially 1350° C. to 1900° C. undera flowing N₂ atmosphere having about ≦0.5% oxygen and about ≦0.5% watervapour;

(b) holding the temperature between about 1350° C. and about 1900° C.for up to about 8 hours; and

(c) recovering the formed product.

Preferably the ceramic material included in the mixture will be coarserthan the other mixture components which react to form the β' orα'SiAlON.

Preferably the ceramic material included in the mixture will constituteup to about 70% by weight of the mixture, and more preferably betweenabout 40% and about 70% by weight of the mixture.

In more limited terms the invention in a fifth aspect comprises aprocess for the production of β'SiAlON having a composition in the range

    Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z

where z has a value between 0.1 and 3.0, from a fine powder componentmixture comprising substantially, by weight, 11%-85% clay, 9%-89%silicon metal and 0%-20% carbon, the process comprising the steps of:

a) heating the components at a rate of between substantially 1° C. and20° C. per minute, to a temperature of substantially 1100° C. to 1750°C. under a flowing nitrogen or nitrogen containing atmosphere havingabout ≦0.5% oxygen and about ≦0.5% water vapour;

b) holding the temperature between about 1100° C. and about 1750° C. forup to about 12 hours; and

c) recovering the product.

Preferably the clay is present in an amount of between 11% and 80% byweight.

Preferably the β'SiAlON formed has a z value in the range of between 0.1and 2.0, more preferably between 0.5 and 1.5, and the reaction isallowed to continue for a time sufficient to form α'SiAlON having acomposition characterised by the general formula:

    M.sub.m/v Si.sub.(m+n) Al.sub.(m+n) O.sub.n N.sub.16-n

where M is a metal cation having a valence v and where m and n indicatethe replacement of (m+n) (Si--N) bonds by m(Al--N) and n(Al--O ) bondsin the α-Si₃ N₄ structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached Figures show a X-ray diffraction patterns of the α' andβ'SiAlON products formed by the process of the invention. In theFigures:

FIG. 1 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 5 and standard β'SiAlON andsilicon nitride patterns;

FIG. 2 shows a graph of β' SiAlON ratio against Y₂ O₃ addition atvarying temperatures.;

FIG. 3 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 12 and a standard XRD patterns;

FIG. 4 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 13 and standard XRD patterns;

FIG. 5 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 14 and standard XRD patterns;

FIG. 6 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 15 and standard XRD patterns;

FIG. 7 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 16 and standard XRD patterns;

FIG. 8 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 17 without zirconia additionand standard XRD patterns;

FIG. 9 shows a comparison between the X-ray diffraction pattern for theproduct formed by Example 17 with zirconia addition and standard XRDpatterns;

FIG. 10 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 18 and standard XRD patterns;

FIG. 11 shows a comparison between the X-ray diffraction pattern for,the product formed by the process of Example 19 and standard XRDpatterns; and

FIG. 12 shows a comparison between the X-ray diffraction pattern for theproduct formed by the process of Example 20 and standard XRD patterns.

The standard α' and β'SiAlON X-ray diffraction pattern was supplied bythe International Centre for Diffraction Data, USA and the product X-raydiffraction patterns were obtained from a Phillips 1700 SeriesDiffractometer controlled by Phillips APD1700 software.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention is a novel process for preparing β'or α'SiAlON from fine powders of silicon metal, clay, (which may containsilica) and, for the production of some compositions, carbon, siliconcarbide, yttria, calcia, magnesia or lithia. The clay may bedehydroxylated prior to use however retention of the clay in its naturalplastic form will allow the mixture to be more readily formed into adesired shape prior to firing. The use of the clay material as a sourceof aluminium and silicon for the production of the SiAlON productionallows the option of utilising the malleable properties of clay to beavailable. The product formed can thus be tailored for a specific useand can be produced very economically.

The β'SiAlON formed by the process of the invention can have a low zvalue of between 0.1 and 3.0. The low z value equates to a low aluminiumcontent in the SiAlON.

When forming α'SiAlON, it is preferable that β'SiAlON having a z valueof between 0.1 and 2.0 is formed first and the reaction is allowed tocontinue to form the α'SiAlON. The z value is more preferably between0.5 and 1.5 in such cases with between 0.5 and 1.0 particularlypreferred.

The raw materials may be blended by standard techniques such as ballmilling or the like as will be known in the art. These raw materials areblended, formed into shapes by traditional methods of pressing, slipcasting, or extruding and more advanced methods including isostaticpressing and injection moulding as will be known in the art, and thenheated under a flowing nitrogen atmosphere to temperatures greater than1400° C. at an appropriate rate, and held at this temperature for up toabout 12 hours, although between 6 and 8 hours is generally seen to besufficient. Longer holding times may be used as will be known in theart. The nitrogen flow rate should be as low as possible, but sufficientto maintain an atmosphere with preferably about ≦0.5% oxygen and about≦0.5% water vapour inside the furnace. The oxygen and water vapourcontent of the atmosphere should be kept to a minimum as they can affectthe process by attacking the silicon. During the reaction the nitrogenfrom the furnace atmosphere becomes incorporated into the product via anitridation reaction giving an increase in density. The product isprimarily α' or β'SiAlON, although O'-SiAlON, mullite and other SiAlONphases, such as X-phase SiAlON, may also be formed.

The carbon and silicon carbide, if used, act as reductants. Siliconmetal may also act as a reductant. Yttria, calcia, magnesia, and otheroxide additives, if used, will stabilise the α'SiAlON structure and willalso assist sintering. Addition of these oxides in the mixture of rawmaterials promotes the reaction, reducing reaction time and temperatureand increasing SiAlON yield.

Some reaction occurs during the heating stage, and therefore the holdingstep for up to 12 hours is optional, however the bulk of the product isformed between 1400° C. and 1450° C. as the silicon begins to melt (atapprox 1414° C.). Holding the furnace at a temperature greater than1450° C. is also optional but may be used to force the reaction tocompletion, or to sinter the body to obtain better densities. A heatingrate of between 1° C. per minute and 5° C. per minute is consideredsuitable however, as may be seen in the Examples, heating rates of up toabout 20° C. per minute may also be used. The preferred temperaturerange is between 1350° C. and 1750° C. as at higher temperaturesspecialised and more expensive kilns may be required. If impurities arepresent in the silicon then the melting point may be lowered. In such acase the reaction may proceed at lower temperatures. Temperatures as lowas about 1100° C. are envisaged as being possible for the preparation ofβ'SiAlON. Such impurities include Fe, Ca and Mg for example as well asothers as will be known in the art. For example in the case of Feparticularly, there is a eutectic between Fe and Si at about 1190° C.The required temperature may preferably be reached by incremental steps.For example the mixture may be heated to between about 80° C. and 130°C. (preferably about 110° C.) and held for up to about 3 hours to drythe mixture. The temperature may then be increased to between about 500°C. and 800° C. (preferably about 600° C.) and held for up to about 3hours to remove the H₂ O formed as the clay dehydroxylates. Thetemperature may then be increased further to between about 950° C. and1300° C. (preferably about 1250° C.) to remove the CO₂ which evolves asthe calcite decomposes. Further incremental steps may be used asrequired in order to optimise the reaction. The holding times arepreferably between 1 and 3 hours. The incremental steps are notessential to the process and may be varied as desired. Any figure orrange of figures within the quoted ranges can be used as will be knownin the art.

As will be readily apparent to a person skilled in the art, the type offurnace or kiln used must be able to maintain a controlled internalatmosphere at the temperatures required. Any type of furnace or kilncapable of this may be used.

It has been found that dehydroxylation of the clay prior to mixing isbeneficial to the production of α' and β'SiAlON in pure form. Forexample, at 500-800° C. clay undergoes this dehydroxylation reaction toform a reactive amorphous intermediate as is well known in the art. Thisdehydroxylation will also occur when the process is carried out in asingle firing (ie without prior dehydroxylation of the clay) thus onestep processing of the material is possible. An example of the amorphousintermediate formed is meta-Kaolin from Kaolin clays. The formation ofthe amorphous intermediate will react with silicon and nitrogen to formsilicon aluminium oxynitrides (SiAlONs) under suitable conditions. Theamorphous intermediate is formed at relatively low temperatures and isreactive at those temperatures which facilitates the use of relativelylow temperatures in the SiAlON forming process.

An example of the reaction to produce β'SiAlON (z=0.5) from New ZealandChina Clays premium grade halloysite is shown in equation (1). Theamount of each raw material must be balanced to provide the correctSi:Al:O:N:C ratio for the desired β'SiAlON as will be known in the art.##STR1##

If the correct balance of raw materials is used then the production ofβ'SiAlON in the resultant ceramic is maximised. This balance of materialwill be able to be calculated readily by a person skilled in the art andwill depend largely on the type of clay used in the reaction.

The process of the present invention allows the manufacture of low zSiAlON compositions (z<3), which have been shown to have desirableproperties, much more readily than traditional methods. High zcompositions (z>3) may also be made by the process of the invention butwith aluminium in the starting mix.

The Industrial Research Limited et al PCT application (PCT/NZ95/00050)discloses the synthesis of O'-SiAlON from mixtures of clay and silicon,assuming no loss of gaseous species from the system. Use of carbon inthe process of the present invention allows carbothermal reduction tooccur which further increases the nitrogen content of the SiAlON productto form β'SiAlON. The use of carbon in the process, while verypreferable, is not essential as can be seen from the Examples herein.

The process of the present invention is capable of producing productscontaining over 80% of β'SiAlON. The percentage of carbon used, byweight, can be as low as 0%. In such a case the proportions, by weight,of clay and silicon metal will be approximately 30% and 70%respectively. The preferred percentage makeup of the staring componentis however and figure or range of figures between about 11% to about 85%(more preferably 80%) clay, about 15% to about 89% silicon metal and 0%to about 20% carbon.

As will be apparent to a person skilled in the art a variety of claysmay be used in process. The preferred clays are hydrated clay mineralsof which the kaolin clays are preferred. Other types of clay may also beused however most will contain a variety of impurities such as K, Na,Ca, Mg, and Fe together with the content. These impurities will affectthe purity of the SiAlON product formed by the process.

As mentioned previously, water vapour inside the furnace due todehydroxylation of the clay during firing can particularly prevent thesilicon reacting. This can be minimised by dehydroxylating the claybefore mixing with the silicon and silica. This may be achieved bymethods such as precalcining as will be known in the art. Sintering aidssuch as Y₂ O₂ can also be added to improve the density (see example 6).In addition Y₂ O₃ can be added to the raw materials to promote thereaction, as can be seen in Example 5.

While the removal of water from the clay prior to firing will enhancethe production of the SiAlON product, the step is not essential to theprocess. Dehydroxylation increases the purity of the product however thebenefits of clay with regard to slip casting and extruding for examplewill no longer be available.

This process can also be used to fabricate composite ceramics, whereβ'SiAlON is used to bond together grains of other ceramic materials suchas silicon carbide (SiC) (see example 5), alumina (Al₂ O₃), siliconnitride (Si₃ N₄), SiAlON, zirconia (ZrO₂) or silica (SiO₂). These bondedmaterials take little or no part in the reaction chemistry. They willpreferably be coarser than the raw materials which react to form thebonding phase, and will preferably constitute between 1 and 70% of thestarting mixture and thus of the fired ceramic. This additional ceramicmaterial is bonded by a matrix of β'SiAlON formed by the othercomponents in the starting mixture (ie the clay, silica, and silicon).These other components will therefore constitute between 30% and 99% ofthe total starting mixture and will be present in the preferredpercentage amounts that have been discussed previously with respect toeach other.

As clay constitutes a significant proportion of the starting mixture,this enables simple and inexpensive forming techniques to be used. Slipcasting, extruding and the like are examples of such techniques. As willbe known in the art more advanced forming methods such as isostaticpressing, injection moulding and the like may also be used. As a resultthere is a great flexibility in the shape and size of ceramic componentswhich can be produced by the process of the present invention.

The reaction to form β'SiAlON is generally accompanied by an amount ofshrinkage, however if β'SiAlON is used to bond another ceramic materialand form a composite ceramic, as mentioned previously, this shrinkagecan become negligible, allowing nearnett size shapes to be formed. Theshapes formed by the forming technique may be of any form as desired.

The method of the present invention is capable of producing eitherceramic bodies or ceramic powder containing the β'SiAlON is a singlefiring step. To make the ceramic powder the reaction proceeds without anemphasis on densification of the resultant ceramic. For example thestarting materials can be formed into pellets, reacted to form a ceramicpellet of β'SiAlON which is then ground into a powder. This may then beused as a supply of β'SiAlON powder for use in other processes. Forexample the powder could be formed and sintered with sintering aids suchas Y₂ O₃, MgO or the like to form fully dense ceramic bodies.

EXAMPLES

The following examples exemplify preferred forms of the invention andare not intended to be limiting.

Example 1

Synthesis of β'SiAlON by Reacting Clay, Silicon, and Carbon withNitrogen

A stoichiometric mixture to form β'SiAlON with z=0.5 was weighed outaccording to the following equation:

    β'SiAlON z=0.5, from NZCC halloysite clay, silicon, and carbon ##STR2## Wt % clay=31.53% Wt % Si=60.79%

Wt % C=7.68% (+10%=8.45%)

Wt Gain=24.06%

Additional carbon (10% of the required amount) was included to coversmall quantities of entrained and physically bound air and water.

The Mixture

6.31 g New Zealand China Clays Premium Grade Halloysite Clay

12.16 g Permascand 4D Silicon

1.69 g Degussa Lampblack 101

The 20 g mixture was blended by ball-milling with approximately 400 g of10 mm diameter Si₃ N₄ balls and 70 g of hexane in a 1 liter high densitypolyethylene (HDPE) bottle for 17 hours at approximately 150 rpm. Thehexane solvent was removed by rotary evaporation. The dry powder wasmade plastic with water and extruded through a 3 mm circular orifice toyield a cylindrical rod which was dried at 110° C. and broken into shortlengths (or pellets) 10-20 mm long.

Two pellets were fired in a horizontal tube furnace (40 mm diam tube) ina small alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 10° C.min⁻¹ to 1350° C., held at thattemperature for 4 hours, then heated at 10° C.min⁻¹ to 1450° C. and heldat that temperature for 8 hours.

The pellets increased in mass by 14% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) revealed primarilyβ'SiAlON with small amounts of O'-SiAlON and traces of silicon carbide.

Example 2

Synthesis of β'SiAlON samples with a range of Aluminium Contents

Stoichiometric mixtures to form β'SiAlON with z=0.25, 0.5, 0.7, 1, and 2were calculated, weighed out, blended, extruded, and fired as describedin Example 1. Analyses of the products by X-ray powder diffraction (XRD)revealed primarily β'SiAlON with small amounts of O'-SiAlON and tracesof silicon carbide. The β'SiAlON content increased with z value, the"z=1" and "z=2" samples consisting entirely of β'SiAlON.

The crystalline phases present were determined by X-ray diffraction. Thealuminium content of the SiAlON (represented by the z value in thegeneral formula Si_(6-z) Al_(z) O_(z) N_(8-z)) was measured by X-raydiffraction using a rapid method developed by Ekstrom [J Mat Sci., 241989 1853.] and adapted at Industrial Research Ltd. The methodcalculates z value from the position of one SiAlON diffraction peak.

Example 3

Use of Silicon Carbide as Reductant

A stoichiometric mixture to form β'SiAlON with z=0.5 was weighed outaccording to the following equation:

    β'Sialon z=0.5, from NZCC halloysite clay, silicon, and silicon carbide Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+13.8Si+15N.sub.2 +5.8SiC-→4Si.sub.5.5 Al.sub.0.5 O.sub.0.5 N.sub.7.5 +5.8CO+2.2H.sub.2 O↑

Additional carbon (10% of the required amount) was included to coversmall quantities of entrained and physically bound air and water.

The 2 g mixture was blended by hand in an agate mortar. The dry powderwas fired in a horizontal tube furnace (40 mm diam tube) in a smallalumina crucible under a flowing nitrogen atmosphere (approximately 100ml.min⁻¹) at 10° C.min⁻¹ to 1350° C., held at that temperature for 4hours, then heated at 10° C.min⁻¹ to 1450° C. and held at thattemperature for 8 hours.

The sample increased in mass during the firing, and an analysis of theproducts by X-ray powder diffraction (XRD) revealed primarily β'SiAlONwith small amounts of O'-SiAlON.

Example 4

Use of yttria to promote synthesis.

A stoichiometric mixture to form β'SiAlON with z=0.5 was weighed outaccording to the recipe given in Example 1 with approximately 0.8%yttria added (calculated as a % of the theoretical final SiAlON mass).

A Mixture of

6.31 g New Zealand China Clays Premium Grade Halloysite Clay

12.16 g Permascand 4D Silicon

1.69 g Degussa Lampblack 101

0.2 g Sigma Yttrium Oxide (Y₂ O₃)

was blended, formed into pellets, and fired as described in Example 1.

The pellets with approximately 0.8% yttria (calculated as a % of thetheoretical final SiAlON mass) increased in mass by 13.2% during thefiring, and an analysis of the products by X-ray powder diffraction(XRD) revealed β'SiAlON alone.

Example 5

Use of yttria to promote synthesis.

The mixture described in Example 1 was prepared in 20 g batches withyttria contents (calculated as a % of the theoretical final SiAlON mass)of 0.5, 0.8, 2.4, 3, and 5%. These were fired in a tube furnace asdescribed in Example 1 for 8 hours at three temperatures 1350, 1400,1450° C. The analysis of the products by X-ray powder diffraction (XRD),as shown in FIG. 1, revealed primarily β'SiAlON at 1450° C. withmixtures of O'-SiAlON and β'SiAlON at lower temperatures. An optimumyttria content of 0.5 to 2.4% is shown in FIG. 2.

Example 6

Use of yttria to promote synthesis and aid sintering.

The mixture described in Example 4 was prepared in 200 g batches andfired in a vertical graphite tube installed in a vacuum sinteringfurnace with graphite heating elements. The same heating schedule wasemployed with a nitrogen flow in a 50 mm diameter tube.

The resulting SiAlON pellets were ball milled for 16 hours in ethanolusing High Density Polyethylene (HDPE) bottles and Si₃ N₄ milling media.

Oleic acid was chosen as a lubricant pressing aid and binder as it couldbe removed completely from the test pieces after forming by heating innitrogen to 450° C. The oleic acid was added to the mill at 5 wt %relative to the total dry powder and the blend remilled for a further 15minutes.

The resultant slurry was filtered through 10 μm cloth to removeagglomerates and a laboratory magnetic stirrer was used to rotate aplastic coated magnet on top of the filter cloth (see diagram page inappendix). This serves to collect magnetic particles and to breakagglomerates, allowing the de-agglomerated material to pass through thecloth.

The slurry with ethanol was dried to near dryness in a rotary evaporatorafter filtering through the 10 micron filter. This was intended tominimise phase separation during drying and prevent airbornecontamination during drying.

The damp powder was granulated through a 500 μm sieve prior tofabrication into 30 mm dia. discs. These discs were uniaxially pressedat up to 35 MPa, and one set of samples was uniaxially pressed at 7 MPaand then pressed in a cold isostatic press at 400 MPa.

The test specimens were heated under N₂ to 450° C. at 0.5° C.min⁻¹ toremove the oleic acid and were then fired under N₂ immersed in a SiAlONpowder bed containing a small proportion of added silica in a ThermalTechnology graphite resistance furnace to 1550° C.-800° C. at 20° C. perminute, held at top temperature for 1 hour, and allowed to coolnaturally.

The discs were composed entirely of β'SiAlON (by XRD), apart from asurface skin, after heating while immersed in a SiAlON powder bedcontaining a small proportion of added silica, for one hour at 1600° C.in a Thermal Technology graphite resistance furnace under a protectivenitrogen atmosphere.

Fired specimens were ground and polished plane parallel into 1.5 mmthick discs suitable for biaxial strength measurement using the ball onring method [4]. Discs >20 mm dia. were supported on a 17 mm dia. ringof steel balls and loaded to failure via a central ball using an Instrontesting machine. The calculated strength varies with the Poisson's ratioof the material. The elastic properties were calculated from the speedof sound measured in the material.

After heating for one hour at 1700° C. under nitrogen the disc exhibitedan apparent porosity of 0.1% and a bulk density of 93% of theoretical.Young's Modulus was found to be 295 GPa and Poisson ratio=0.25. Thesefigures are comparable to those obtained for the z=2.7 SiAlON made fromclay and carbon alone. With increasing porosity the Young's modulusremained constant but the strength increased to 583 MPa at 0.37%apparent porosity.

Example 7

Silicothermic Reduction--use of silicon metal as reductant

A stoichiometric mixture to form β'SiAlON with z=0.5 was weighed outaccording to the following equation:

    β'SiAlON z=0.5, from NZCC halloysite clay and silicon

    Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+25.4Si+15N.sub.2 -→4Si.sub.5.5 Al.sub.0.5 O.sub.0.5 N.sub.7.5 +5.8SiO+2.2H.sub.2 O↑

Wt % clay 28.6%

Wt % Si=71.4%

Wt Gain=12.5%

The Mixture

0.572 g New Zealand China Clays Premium Grade Halloysite Clay

1.428 g Permascand 4D Silicon

The 2 g mixture was blended by hand in an agate mortar. An 0.4 g disc(10 mm diam.) was formed from the powder mixture by uniaxial pressing at8 MPa pressure and was fired in a horizontal tube furnace (40 mm diamtube) in a small alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 10° C.min⁻¹ to 1350° C., held at thattemperature for 4 hours, then heated at 10° C.min⁻¹ to 1450° C. and heldat that temperature for 8 hours.

The sample increased in mass by 23.6% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) revealed primarilyβ'SiAlON (z=0.2) with a small quantity of a silicon nitride.

Example 8

Silicothermic Reduction with yttria additive.

A stoichiometric mixture to form β'SiAlON with z=0.5 was weighed outaccording to the equation given in Example 7 above and 3 weight %yttrium oxide was added. A sample disc was formed from this mixture andfired as described in Example 7.

The sample increased in mass by 17% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) revealed only β'SiAlON(z=0.42).

Example 9

Reaction bonding β'SiAlON.

The mixture described in Example 5 with 3% yttria content (calculated asa % of the theoretical final SiAlON mass) was prepared in 2 g batches.The 2 g mixture was blended by hand in an agate mortar. An 0.4 g disc(10 mm diam.) was formed from the powder mixture by uniaxial pressing at8 MPa pressure and was fired in a horizontal tube furnace (40 mm diamtube) in a small alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 10° C.min⁻¹ to 1100° C., then at 1°C.min⁻¹ to 1450° C., and held at that temperature for 12 hours, thenallowed to cool naturally.

The sample increased in mass by 16.7% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) revealed β'SiAlON(z=0.2).

The bulk density and open porosity of the fired pellets were measured byimmersion in water:

bulk density=1.6 g.cm³

apparent porosity=46.2%

Example 10

Reaction bonding silicon carbide with β'SiAlON.

A Mixture of

60% Navarro 36-grit silicon carbide (SiC)

40% the mixture from Example 9

The 2 g mixture was blended by hand in an agate mortar. An 0.4 g disc(10 mm diam.) was formed from the powder mixture and fired as describedfor Example 9.

The sample increased in mass by 12.9% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) revealed primarily SiCand β'SiAlON. The bulk density and open porosity of the fired pelletswere measured by immersion in water:

bulk density=2.1 g.cm³

apparent porosity=31.1%

Example 11

Synthesis of α'SiAlON in one step.

A Mixture of

1.063 g New Zealand China Clays Premium Grade Halloysite Clay

0.655 g Permascand 4D Silicon

0.348 g Degussa Lampblack 101

0.19 g Sigma Yttrium Oxide (Y₂ O₃)

The powder mixture was blended by hand in an agate mortar and fired insmall alumina crucibles under N₂ immersed in a Thermal Technologygraphite resistance furnace to 1600° C. at 20° C. per minute, held attop temperature for 1 hour, and allowed to cool naturally.

Analysis of the products by X-ray powder diffraction (XRD) revealedprimarily α'SiAlON with β'SiAlON and melilite (ICDD JCPDS 28-1457) alsopresent.

Example 12

Synthesis of Ca α'SiAlON by reacting calcite, clay, silicon, and carbonwith nitrogen.

A stoichiometric mixture to form Ca α'SiAlON with m=1.5, n=0.75 wasweighed out according to the following equation:

    Silicothermal/carbothermal reduction and nitridation to form Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 0.75CaCO.sub.3 +1.125Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+7.05Si+8.78C-→Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 +8.78CO+0.75CO.sub.2 +2.475H.sub.2 O

Wt % calcite=6.86%

Wt % clay=29.36%

Wt % Si=38.70%

Wt % C=25.08% (+10%=27.59%)

Wt Loss=15.7%

Additional carbon (10% of the required amount) was included to coversmall quantities of entrained and physically bound air and water.

The Mixture

2.54 g BDH Chemicals Ltd. Analytical Reagent Calcite

10.89 g New Zealand China Clays Premium Grade Halloysite Clay

6.71 g Permascand 4D Silicon

3.925 g Degussa Lampblack 101

The mixture was blended by ball-milling with approximately 400 g of 10mm diameter Si₃ N₄ balls and 70 g of hexane in a 1 liter high densitypolyethylene (HDPE) bottle for 17 hours at approximately 150 rpm. Thehexane solvent was removed by rotary evaporation. The dry powder wasuniaxially pressed to 8 MPa in a 13 mm diameter steel die to formpellets approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 mi.min⁻¹) at 5° C.min⁻¹ to 600° C., held at thattemperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. and held atthat temperature for 2 hours, then heated at 5° C.min⁻¹ to 1550° C. andheld at that temperature for 7 hours, then cooled at 10° C.min⁻¹ untilthe natural cooling rate of the furnace was slower than 10° C.min⁻¹after which it was allowed to cool to room temperature and the samplewas recovered.

The pellet decreased in mass by 22.5% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) as shown in FIG. 4,revealed primarily Ca α'SiAlON with small amounts of β'SiAlON as shownin the attached XRD pattern in FIG. 3.

Example 13

Synthesis of Ca α'SiAlON by reacting calcite, clay, and silicon, withnitrogen.

A stoichiometric mixture to form Ca α'SiAlON with m=1.5, n=0.75 wasweighed out according to the following equation :

    Silicothermal reduction and nitridation to form Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 0.75CaCO.sub.3 +1.125Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+7.05Si+8.78Si®-→Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 +8.78SiO+0.75CO.sub.2 +2.475H.sub.2 O

The silicon expected to form SiO vapour is expressed separately as Si®.

Wt % calcite=8.93%

Wt % clay=38.21%

Wt % Si=23.55%

Wt % Si®=29.31%

Total % Si=52.86%

Wt Loss=29.8%

The Mixture

2.54 g BDH Chemicals Ltd. Analytical Reagent Calcite

10.89 g New Zealand China Clays Premium Grade Halloysite Clay

15.065 g Permascand 4D Silicon

The mixture was blended by ball-milling with approximately 400 g of 10mm diameter Si₃ N₄ balls and 70 g of hexane in a 1 liter high densitypolyethylene (HDPE) bottle for 17 hours at approximately 150 rpm. Thehexane solvent was removed by rotary evaporation. The dry powder wasuniaxially pressed to 8 MPa in a 13 mm diameter steel die to formpellets approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 5° C.min⁻¹ to 110° C., held at thattemperature for 2 hours, then heated at 5° C.min⁻¹ to 600° C., held atthat temperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. andheld at that temperature for 2 hours, then heated at 1° C.min⁻¹ to 1250°C. and held at that temperature for 4 hours, then heated at 1° C.min⁻¹to 1400° C. and held at that temperature for 8 hours, then heated at 5°C.min⁻¹ to 1650° C. and held at that temperature for 8 hours, thencooled at 10° C.min⁻¹ until the natural cooling rate of the furnace wasslower than 10° C.min⁻¹ after which it was allowed to cool to roomtemperature and the sample recovered.

The pellet decreased in mass by 33.8% during the firing, and an analysisof the products by X-ray powder diffraction (XRD), as shown in FIG. 4,revealed primarily Ca α'SiAlON with a small amount of β'SiAlON.

Example 14

Synthesis of Ca α'SiAlON by reacting calcite, clay, silicon and siliconcarbide, with nitrogen.

A stoichiometric mixture to form Ca α'SiAlON with m=1.5, n=0.75 wasweighed out according to the following equation:

    Silicothermal/carbothermal reduction and nitridation to form Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 0.75CaCO.sub.3 +1.125Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+7.05Si+4.39SiC-→Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 +4.39SiO+4.39CO+2.475H.sub.2 O+0.75CO.sub.2

The silicon carbide was included as a reductant.

Wt % calcite 9.86%

Wt % clay=42.21%

Wt % Si=21.76%

Wt % SiC=21.76% (+10%=23.93%).

Wt Loss=22.4%

The Mixture

0.13 g BDH Chemicals Ltd. Analytical Reagent Calcite

0.545 g New Zealand China Clays Premium Grade Halloysite Clay

0.309 g Navarro #1000 C5 Silicon Carbide

0.336 g Permascand 4D Silicon

The mixture was blended by dry grinding by hand in an agate mortar. Thedry powder was uniaxially pressed to 8 MPa in a 13 mm diameter steel dieto form pellets approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 5° C.min⁻¹ to 110° C., held at thattemperature for 2 hours, then heated at 5° C.min⁻¹ to 600° C., held atthat temperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. andheld at that temperature for 2 hours, then heated at 1° C.min⁻¹ to 125°C. and held at that temperature for 4 hours, then heated at 1° C.min⁻¹to 1400° C. and held at that temperature for 8 hours, then heated at 5°C.min⁻¹ to 1650° C. and held at that temperature for 8 hours, thencooled at 10° C.min⁻¹ until the natural cooling rate of the furnace wasslower than 10° C.min⁻¹ after which it was allowed to cool to roomtemperature and the sample recovered.

The pellet decreased in mass by 36.7% during the firing, and an analysisof the products by X-ray powder diffraction (XRD) as shown in FIG. 5,revealed Ca α'SiAlON.

Example 15

Synthesis of a composite of with Li α'SiAlON with β'SiAlON by reactingspodumene (LiAlSi₂ O₆), clay, silicon, and carbon with nitrogen.

A stoichiometric mixture to form Li α'SiAlON with m=1.5, n=0.75 wasweighed out according to the following equation:

    Silicothermal/carbothermal reduction and nitridation to form Li.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 1.5LiAlSi.sub.2 O.sub.6 +0.375Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+5.85Si+11.18C-→Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 +11.18CO0.825H.sub.2 O

Wt % spodumene=41.47%

Wt % clay=15.91%

Wt % Si=24.41%

Wt % C=18.32% (+10%=20.16%)

Wt Loss=15.4%

Additional carbon (10% of the required amount) was included to coversmall quantities of entrained and physically bound air and water.

The Mixture

0.49 g Spodumene

0.188 g New Zealand China Clays Premium Grade Halloysite Clay

0.288 g Permascand 4D Silicon

0.238 g Degussa Lampblack 101

The mixture was blended by dry grinding by hand in an agate mortar. Thedry powder was uniaxially pressed to 8 MPa in a 13 mm diameter steel dieto form pellets approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 5° C.min⁻¹ to 110° C., held at thattemperature for 2 hours, then heated at 5° C.min⁻¹ to 600° C., held atthat temperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. andheld at that temperature for 2 hours, then heated at 1° C.min⁻¹ to 1250°C. and held at that temperature for 4 hours, then heated at 1° C.min⁻¹to 1400° C. and held at that temperature for 8 hours, then heated at 5°C.min⁻¹ to 1650° C. and held at that temperature for 8 hours, thencooled at 10° C.min⁻¹ until the natural cooling rate of the furnace wasslower than 10° C.min⁻¹ after which it was allowed to cool to roomtemperature and the sample recovered.

The pellet decreased in mass by 32.9% during the firing, and an analysisof the products by X-ray powder diffraction (XRD), as shown in FIG. 6,revealed primarily β'SiAlON with Li α'SiAlON present in smallerproportion. The result was effectively a composite of with Li α'SiAlONwith β'SiAlON.

Example 16

Synthesis of a composite of Y α'SiAlON with β'SiAlON by reacting yttria,clay, silicon, and carbon with nitrogen.

A stoichiometric mixture to form Yα'SiAlON with m=1.5, n=0.75 wasweighed out according to the following equation:

    Silicothermal/carbothermal reduction and nitridation to form Ca.sub.0.75 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 0.25Y.sub.2 O.sub.3 +1.125Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+7.05Si+8.78C-→Y.sub.0.5 Si.sub.9.75 Al.sub.2.25 O.sub.0.75 N.sub.1.5 +8.78CO+2.475H.sub.2 O

Wt % yttria=5.25%

Wt % clay=29.86%

Wt % Si=39.37%

Wt % C=25.51% (+10%=28.06%)

Wt Loss=43.8%

Additional carbon (10% of the required amount) was included to coversmall quantities of entrained and physically bound air and water.

The Mixture

1.87 g HC Starck fine grade Lot 1/94 Yttrium Oxide

10.63 g New Zealand China Clays Premium Grade Halloysite Clay

14.015 g Permascand 4D Silicon

9.981 g Degussa Lampblack 101

The mixture was blended by ball-milling with approximately 400 g of 10mm diameter Si₃ N₄ balls and 70 g of hexane in a 1 liter high densitypolyethylene (HDPE) bottle for 17 hours at approximately 150 rpm. Thehexane solvent was removed by rotary evaporation. The dry powder wasuniaxially pressed to 8 MPa in a 13 mm diameter steel die to formpellets approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 5° C.min⁻¹ to 600° C., held at thattemperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. and held atthat temperature for 2 hours, then heated at 5° C.min⁻¹ to 1450° C. andheld at that temperature for 7 hours, then cooled the natural coolingrate of the furnace to room temperature and the sample recovered.

The pellet decreased in mass by 16.6% during the firing, and an analysisof the products by X-ray powder diffraction (XRD), as shown in FIG. 7,revealed Y α'SiAlON with an approximately equal amount of β'SiAlON.

Example 17

The effect of small amounts of additives on the synthesis of α'SiAlON.

The addition of small amounts (2 volume %) of some metal oxides promotethe reaction. The addition of 2 volume % of zirconia, yttria, cuprousoxide, and hematite each separately promoted the formation of α'SiAlONfrom the mixtures described in Examples 12 and 13.

To a subsample taken from the 20 g mixture described in Example 12. azirconia addition equivalent to 2% by volume was made.

The Mixture

0.010 g Tosoh TZ-0 Zirconia ZrO₂

0.337 g Mixture A21 (Calcite+Clay+Silicon+Carbon) ex Example 12

The mixture was blended by dry grinding by hand in an agate mortar. Thedry powder was uniaxially pressed to 8 MPa in a 13 mm diameter steel dieto form pellets approximately 0.3 g. in weight.

The pellet (21A13) was fired with a similar pellet formed from MixtureA21 (2113) (Calcite+Clay+Silicon+Carbon ex Example 12) in a horizontaltube furnace (40 mm diam tube) in a small alumina crucible under aflowing nitrogen atmosphere (approximately 100 ml.min⁻¹) at 5° C.min⁻¹to 110° C., held at that temperature for 2 hours, then heated at 5°C.min⁻¹ to 600° C., held at that temperature for 1 hour, then heated at5° C.min⁻¹ to 1400° C. and held at that temperature for 8 hours, thencooled at 10° C.min⁻¹ until the natural cooling rate of the furnace wasslower than 10° C.min⁻¹ after which it was allowed to cool to roomtemperature and the samples recovered.

The pellet 21A13 with the zirconia addition decreased in mass by 24.0%during the firing, and the pellet 2113 with no zirconia additiondecreased in mass by 24.8% during the firing. An analysis of theproducts by X-ray powder diffraction (XRD) revealed Ca α'SiAlON andβ'SiAlON present in both samples with approximately twice the Caα'SiAlON content in the pellet 21A13 with the zirconia addition.

FIG. 8 shows the XRD pattern--no zirconia addition.

FIG. 9 shows the XRD pattern--with zirconia added.

Example 18

The effect of large additions of ceramic additives on the synthesis ofα'SiAlON.

To a subsample taken from the 20 g mixture described in Example 12. analuminium nitride addition equivalent to 50% by volume was made.

The Mixture

0.149 g Aluminium Nitride HC Starck

0.198 g Mixture A21 (Calcite+Clay+Silicon+Carbon) ex Example 12

The mixture was blended by dry grinding by hand in an agate mortar. Thedry powder was uniaxially pressed to 8 MPa in a 13 mm diameter steel dieto form a pellet approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 5° C.min⁻¹ to 110° C., held at thattemperature for 2 hours, then heated at 5° C.min⁻¹ to 600° C., held atthat temperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. andheld at that temperature for 2 hours, then heated at 5° C.min⁻¹ to 1100°C., then heated at 1° C.min⁻¹ to 1250° C. and held at that temperaturefor 4 hours, then heated at 1° C.min⁻¹ to 1450° C. and held at thattemperature for 8 hours, then cooled at 10° C.min⁻¹ until the naturalcooling rate of the furnace was slower than 10° C.min⁻¹ after which itwas allowed to cool to room temperature and the sample recovered.

The pellet increased in mass by 3.9% during the firing, and an analysisof the products by X-ray powder diffraction (XRD), as shown in FIG. 10,revealed equal amounts of Ca α'SiAlON and AlN.

Example 19

The effect of large additions of coarse ceramic additives on thesynthesis of α'SiAlON. Silicon carbide is the coarse ceramic additiveemployed here and this demonstrates the potential for the manufacture ofa reaction bonded composite product.

To a subsample taken from the 20 g mixture described in Example 12. asilicon carbide addition equivalent to 50% by volume was made.

The Mixture

0.148 g Navarro 80 mesh C5 Silicon Carbide

0.199 g Mixture A21 (Calcite+Clay+Silicon+Carbon) ex Example 12

The mixture was blended by dry grinding by hand in an agate mortar. Thedry powder was uniaxially pressed to 8 MPa in a 13 mm diameter steel dieto form a pellet approximately 0.3 g. in weight.

The pellet was fired in a horizontal tube furnace (40 mm diam tube) in asmall alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 5° C.min⁻¹ to 110° C., held at thattemperature for 2 hours, then heated at 5° C.min⁻¹ to 600° C., held atthat temperature for 1 hour, then heated at 5° C.min⁻¹ to 800° C. andheld at that temperature for 2 hours, then heated at 5° C.min⁻¹ to 1100°C., then heated at 1° C.min⁻¹ to 1250° C. and held at that temperaturefor 4 hours, then heated at 1° C.min⁻¹ to 1450° C. and held at thattemperature for 8 hours, then cooled at 10° C.min⁻¹ until the naturalcooling rate of the furnace was slower than 10° C.min⁻¹ after which itwas allowed to cool to room temperature and the sample recovered.

The pellet decreased in mass by 12.3% during the firing, and an analysisof the products by X-ray powder diffraction (XRD), as shown in FIG. 11,revealed equal amounts of Ca α'SiAlON and silicon carbide with a smallamount of β'SiAlON.

Example 20

Synthesis of β'SiAlON composites with β'SiAlON by reacting clay,silicon, and carbon with nitrogen.

A stoichiometric mixture to form β'SiAlON with z=0.5 was weighed outaccording to the following equation:

    β'SiAlON z=0.5, from NZCC halloysite clay, silicon, and carbon Al.sub.2 O.sub.3 2.4SiO.sub.2.2.2H.sub.2 O+19.6Si+15N.sub.2 +5.8C-→4Si.sub.5.5 Al.sub.0.5 O.sub.0.5 N.sub.7.5 +5.8CO+2.2H.sub.2 O↑

Wt % clay=31.53%

Wt % Si=60.79%

Wt % C=7.68% (+10%=8.45%)

Wt Gain=24.06%

Additional carbon (10% of the required amount) was included to coversmall quantities of entrained and physically bound air and water.

The Mixture

6.31 g New Zealand China Clays Premium Grade Halloysite Clay

12.16 g Permascand 4D Silicon

1.69 g Degussa Lampblack 101

The 20 g mixture was blended by ball-milling with approximately 400 g of10 mm diameter Si₃ N₄ balls and 70 g of hexane in a 1 liter high densitypolyethylene (HDPE) bottle for 17 hours at approximately 150 rpm. Thehexane solvent was removed by rotary evaporation. The dry powder wasmade plastic with water and extruded through a 3 mm circular orifice toyield a cylindrical rod which was dried at 110° C. and broken into shortlengths (or pellets) 10-20 mm long.

Two pellets were fired in a horizontal tube furnace (40 mm diam tube) ina small alumina crucible under a flowing nitrogen atmosphere(approximately 100 ml.min⁻¹) at 10° C.min⁻¹ to 1350° C., held at thattemperature for 4 hours, then heated at 10° C.min⁻¹ to 1400° C. and heldat that temperature for 8 hours.

The pellets increased in mass by 22.3% during the firing, and ananalysis of the products by X-ray powder diffraction (XRD), as shown inFIG. 12, revealed approximately equal amounts of β'SiAlON and O'-SiAlONand traces of silicon carbide.

Example 21

Synthesis of other SiAlON compounds with β'SiAlON.

Given the parameters of the invention a synthesis employing thefollowing compositions will form composites of β'SiAlON with otherSiAlON compounds by this process.

Mixture 1

Clay 72.84%

Silicon 11.90%

Carbon 15.26%

This mixture when reacted by the process described here will form about87% β'SiAlON and about 13% X phase SiAlON.

Mixture 2

Clay 55.33%

Silicon 39.30%

Carbon 5.17%

This mixture when reacted by the process described here will form about47% β'SiAlON, about 47% β'SiAlON, and 6% X phase SiAlON.

The foregoing describes preferred forms of the invention and it is to beunderstood that the scope of the invention is not to be limited to thespecific forms described. Modifications and variations as will beobvious to a person skilled in the art may be made to the forms of theinvention as described without departing from the spirit or scope of theinvention as defined in the attached claims.

What is claimed is:
 1. A process for producing a ceramic materialcomprising α'SiAlON from a starting mixture comprising silicon metal,clay and a source of a metal cation capable of stabilizing the structureof α'SiAlON, wherein the process comprises heating the starting mixturein a flowing nitrogen, or nitrogen containing, atmosphere to atemperature sufficient to react the silicon metal and the nitrogen withthe clay to form the α'SiAlON and wherein the clay participates in thereaction as a source of aluminium and silicon.
 2. A process forpreparing a ceramic material comprising α' or β'SiAlON from a startingmixture comprising silicon metal, clay and silicon carbide, wherein theprocess comprises heating said mixture in a flowing nitrogen, ornitrogen-containing, atmosphere to a temperature sufficient to reactsaid silicon metal and said nitrogen with said clay to form said α' orβ'SiAlON and wherein said clay participates in the reaction as a sourceof aluminium and silicon.
 3. The process of claim 1 or claim 2 whereinthe clay is dehydroxylated clay.
 4. The process of claim 1 or claim 2wherein the clay in the starting mixture is present between about 11%and about 80% by weight.
 5. The process of claim 1 wherein the startingmixture further includes an additive selected from carbon and siliconcarbide.
 6. The process of claim 1 wherein the starting mixturecontains, by weight, about 11% to about 80% clay, about 9% to about 89%silicon metal and 0% to about 20% carbon.
 7. The process of claim 1 orclaim 2 wherein said source of a metal cation is a sintering aidselected from the group consisting of yttria, calcia, magnesia andlithia.
 8. The process of claim 1 or claim 2 wherein the clay contains afree silica component.
 9. The process of claim 1 wherein the siliconmetal and said clay are present as fine powders.
 10. The process ofclaim 1 or claim 2 wherein the atmosphere is selected from the groupconsisting of substantially pure nitrogen, a mixture of hydrogen andnitrogen, and ammonia.
 11. The process of claim 1 or claim 2 wherein theatmosphere is a flowing nitrogen atmosphere which comprises about 0.5%oxygen or less and about 0.5% water vapor or less.
 12. The process ofclaim 1 or claim 2 wherein the mixture is heated to between about 1350°C. and about 1900° C.
 13. The process of claim 12, wherein said mixtureis heated at a rate of from about 1° C. to about 10° C. per minute. 14.The process of claim 13, wherein said mixture is held at a temperaturesufficient to react the silicon and the nitrogen with the clay, for aperiod of up to about 12 hours.
 15. The process of claim 1 or claim 2wherein the clay is a hydrated clay.
 16. The process of claim 15 whereinthe hydrated clay is a Kaolin clay.
 17. The process of claim 1 whereinthe starting mixture further includes a ceramic material, said ceramicmaterial being selected from the group consisting of silicon carbide(SiC), alumina (Al₂ O₃), aluminium nitride (AlN), silicon nitride (Si₃N₄), SiAlON, zirconia (ZrO₂) and silica (SiO₂).
 18. The process of claim17 wherein the ceramic material is coarser than each of the siliconmetal, the clay or the source of a metal cation.
 19. The process ofclaim 17 wherein the ceramic material constitutes from about 40% toabout 70% by weight of the starting mixture.
 20. The process of claim 1wherein the SiAlON formed is an α'SiAlON having a formula:

    M.sub.m/v Si.sub.(m+n) Al.sub.(m+n) O.sub.n N.sub.16-n

where M is a metal cation having a valence v and where m and n indicatethe replacement of (m+n) (Si--N) bonds by m(Al--N) and n(Al--O) bonds inthe α-Si₃ N₄ structure.
 21. The process of claim 20 wherein the α'SiAlONis formed from a β'SiAlON having a formula:

    Si.sub.6-z Al.sub.z O.sub.z n.sub.8-z

where z is in a range of 0.1-3.0.
 22. The process of claim 2, whereinsaid starting mixture includes, by weight, about 11% to about 80% clayand about 9% to about 89% silicon metal.
 23. The process of claim 2wherein the silicon metal, clay and silicon carbide are present as finepowders.
 24. The process of claim 2 wherein the mixture is held at atemperature sufficient to react the silicon metal and nitrogen with theclay, for a period of up to about 12 hours.
 25. The process of claim 2wherein the clay is a hydrated clay.
 26. The process of claim 25 whereinthe hydrated clay is a Kaolin clay.
 27. The process of claim 2 whereinthe starting mixture further includes a ceramic material, said ceramicmaterial being selected from the group consisting of silicon carbide(SiC), alumina (Al₂ O₂), aluminium nitride (AlN), silicon nitride (Si₃N₄), SiAlON, zirconia (ZrO₂) or silica (SiO₂).
 28. The process of claim27 wherein the ceramic material is coarser than each of the siliconmetal, the clay and the silicon carbide.
 29. The process of claim 27wherein the ceramic material constitutes from about 40% to about 70% byweight of the starting mixture.
 30. The process of claim 2 wherein theSiAlON formed as β'SiAlON has a composition of a general formula ofSi_(6-z) Al_(z) O_(z) M_(8-z) wherein z is in the range of from 0.1 to3.0.
 31. A process for producing a composite ceramic including β' orα'SiAlON from a mixture of fine powder components comprising a ceramicmaterial and a β' or α'SiAlON precursors, wherein said precursorsinclude silicon metal and clay, and wherein said process comprisesheating said mixture in a flowing nitrogen, or nitrogen-containing,atmosphere to a temperature sufficient to react said silicon metal andsaid nitrogen with said clay to form β' or α'SiAlON, such that acomposite ceramic including β' or α'SiAlON is formed, wherein said clayparticipates in the reaction as a source of aluminium and silicon.
 32. Aprocess according to claim 31, wherein said ceramic material comprisesfrom about 40% to about 70% by weight of said mixture, and saidprecursors comprise, by weight, about 50% to about 70% silicon metal,about 20% to about 40% clay and about 5% to about 10% carbon.
 33. Aprocess according to claim 32, wherein said mixture is heated at a rateof from about 1.5° C. to about 10° C. per minute, to a temperature offrom about 1350° C. to about 1900° C. in said atmosphere, wherein saidatmosphere has about 0.5% oxygen or less and about 0.5% water vapor orless, and the temperature is held at between about 1350° C. to about1900° C. for a period of up to about eight hours to form said compositeceramic.
 34. The process of claim 33, wherein said ceramic materialincluded in the mixture is coarser than each of said precursors.
 35. Theprocess of claim 34, wherein said ceramic material is selected from thegroup consisting of silicon carbide (SiC), alumina (Al₂ O₃), aluminiumnitride (AlN), silicon nitride (Si₃ N₄), SiAlON, zirconia (ZrO₂) andsilica (SiO₂).